1. Field of the Invention
This invention relates to an automatic transmission control system for an automobile, and, in particular, to an automobile automatic transmission system of the type having specific friction coupling elements simultaneously engaged or applied and disengaged or released to provide a specific gear.
2. Description of the Related Art
Automatic transmissions for automotive vehicles, which typically comprise a torque converter and a transmission gear mechanism, are automatically shifted into desired gears by selectively engaging or applying and disengaging or releasing a plurality of friction coupling elements such as clutches and brakes. This type of automatic transmission is provided with a hydraulic control circuit to supply/discharge hydraulic fluid to these friction coupling elements. As shown in, for example, Japanese Unexamined Patent Publication No. 7-119820, a band-brake type of friction coupling element which cooperates with a servo cylinder as an actuator is used together with ordinary friction coupling elements which are engaged with hydraulic fluid. This servo cylinder actuator having a servo apply fluid chamber and a servo release fluid chamber separated by a spring loaded piston applies a brake when only the apply chamber is supplied with hydraulic fluid and release a brake when both apply chamber and the release chamber are supplied with fluid chamber or when neither the apply chamber nor the release chamber are supplied with any hydraulic fluid. In the case where the frictional coupling element operated by the servo cylinder actuator is a so-called 2-4 brake which is applied in a second speed gear or in a fourth speed gear, during a shift from a first speed gear to the second speed gear, the servo cylinder actuator with both chambers remaining empty is caused to apply the 2-4 brake when receiving hydraulic fluid in the apply chamber in a state where both chambers are empty is supplied. On the other hand, during a shift from a third speed gear to the second speed gear or the fourth speed gear, the servo cylinder actuator with both chambers remaining supplied with fluid chamber is caused to apply the 2-4 brake when discharging hydraulic fluid from the release chamber in a state where both chambers are empty is supplied.
In the automatic transmission of this type, a specific gear needs to disengage one friction coupling element simultaneously with engaging another specific gear. For example, during a down-shift from the third speed gear to the second speed gear (a 3-2 shift), while the 2-4 brake is applied by discharging hydraulic fluid from the release chamber of the servo cylinder actuator, a 3-4 clutch, which is engaged for the third speed gear and the fourth speed gear, is disengaged by discharging hydraulic fluid from its fluid chamber. During an up-shift from the third speed gear to the fourth speed gear (a 3-4 shift), while the 2-4 brake is applied by discharging hydraulic fluid from the release chamber of the servo cylinder actuator, a forward clutch, which is engaged for the first, second and third speed gears, is disengaged by discharging hydraulic fluid from its fluid chamber. In such a case, since discharging hydraulic fluid from the release chamber of the 2-4 brake and the fluid chamber of the 3-4 clutch or the forward clutch, simultaneously, these release chamber and the fluid chambers are communicated to discharge hydraulic fluid under operation of a fluid control valve, such as a duty solenoid valve.
In the case of a torque demand 3-2 shift following an increase in engine load, and a manual 3-2 shift with the engine remaining loaded, in order to cause an increase in turbine speed smoothly during the gear shift, it is always desired to feedback control engaging force of the 3-4 clutch according to a deviation from an instantaneous target speed. In such a case, the utilization may be made of controlling hydraulic fluid in the apply chamber of the 2-4 brake to perform the feedback control. Specifically, since the release chamber of the 2-4 brake is adjacent to the apply chamber of the 2-4 brake through the piston and is in communication with the fluid chamber of the 3-4 clutch,. the hydraulic fluid for engaging the 3-4 clutch is controlled by means of control of hydraulic fluid in the apply chamber of the 2-4 brake. Accordingly, the utilization may be made of a first and a second pressure control valve to control the pressure of hydraulic fluid in the servo apply fluid chamber of the 2-4 brake and the amount of hydraulic fluid discharged from the release chamber of the 2-4 brake and the fluid chamber of the 3-4 clutch, respectively, with an effect of increasing accuracy of the feedback control.
A torque demand 3-2 shift is made with a driver's intention to accelerate. While there is, accordingly, a strong demand for increasing the responsiveness of the transmission which is achieved by accelerating release of the 3-4 brake, it is required for the 2-4 brake to operate smoothly so as to avoid shift shocks, which are somewhat technically conflicting requirements. A manual 3-2 shift which is tried with the engine throttle held fully closed to apply engine braking or a 3-4 shift caused by fully closing the engine throttle is caused not by disengaging the 3-4 clutch or the forward clutch but engaging mainly the 2-4 brake. In order to achieve these shifts as quickly as possible, the 2-4 brake has the necessity to satisfy those two conflicting requirements.